1. Field of the Invention
This invention relates to Personal Computers (PC's), and more particularly to adapters for connecting a redundant disk to the Parallel Port of a PC.
2. Description of the Related Art
Personal computers (PC's) are a vital part of most businesses today. While larger companies often use large mainframes for critical data, smaller firms often use inexpensive PC's even for critical data. The cost of PC's have fallen so much that the firm's data stored on the PC can be more valuable than the PC itself. Customer lists, work in progress, commercial and proprietary software are some examples.
The mantra of the expert's advice to these firms is "backup, backup, backup." Tape drives for backing up the PC's data are commonly used today to offer a degree of protection from data disaster. Unfortunately, these tape drives are slow in comparison to the hard-disk drives the primarily store the data, and users may perform a backup of their data only once a day, week, or month. The user often has to run specialized backup software, or a network administrator has to set up a backup schedule for networked PC's.
Redundant hard-disk drives are becoming increasingly popular, since data is backed up to the redundant hard disk immediately each time data is written to the primary hard disk. Thus no data is lost. However, these redundant disk arrays (RAID) are typically difficult to install, being mounted inside the PC's chassis. RAID disks installed inside the PC's chassis are not easily transportable. The complexities of installing a RAID disk in a PC can discourage many potential users.
Disks can also be installed outside of the PC's chassis. One desirable feature of PC's are the industry-standard busses and communication ports, allowing easy expansion by the end user. Early PC's had a serial port and a parallel port, usually located on the rear panel of the chassis. The serial port was used to communicate data in a serial fashion to external devices such as modems and mice, while the parallel port was used to communicate 8 bits of data in parallel to a printer. The connection of an external printer to the parallel port is so common that it is also known as the printer port.
The types and number of external devices that connect to the serial and parallel ports of a PC has grown over the years. While once just a printer was attached to the parallel port, now many users have two printers--a high-quality laser printer and a draft-mode dot-matrix printer. Other devices have also appeared that connect to the parallel port. These devices include external disk drives, tape drives, CD-ROM drives, and local-area network adapters. Data transfer to a portable PC such as a Laptop or Notebook can occur over a special cross-wired serial or parallel cable that connects a desktop PC to the portable PC.
Disks attached to the PC's parallel port are common today, but they operate as an additional disk drive rather than as a redundant disk drive. These disks do not mirror the writes to the PC's internal hard disk and thus do not provide redundancy. Standard PC software drivers and BIOS do not provide for redundancy. Configuring the parallel port and disk is complex because addresses and interrupts may have conflicts with other expansion devices on the PC. Often jumpers are used to select the addresses or interrupts for an expansion card.
When the primary disk fails, the PC must be re-started or booted from another disk. Often the removable diskette or floppy disk drive is used. When the data is backed up to the disk on the parallel port, the software driver for the parallel-port disk must also be loaded to memory, perhaps requiring that the boot diskette be swapped out for the parallel-port driver diskette. Swapping diskettes is undesirable, and the diskettes are easy to lose.
It is thus desirable to boot the computer from a hard disk on the parallel port. It is especially desirable to boot the PC from a redundant disk on the parallel port. It is desired to automatically mirror writes to the primary hard disk to the parallel-port disk so that the parallel-port disk is a fully redundant back-up disk. It is further desired to automatically configure the address of the parallel-port controller.